1. Technical Field
The present invention relates to an image projection apparatus and a method of controlling a light source, and in particular, to an image projection apparatus and a method of controlling a light source in which a laser is used as the light source.
2. Background Art
In recent years, projectors using a laser as a light source has been actively developed. The laser used as a light source is better than a lamp, as a light source, in the following respects. The size and weight of the light source itself can be easily reduced. Projection efficiency can be enhanced due to the small etendue of the laser. The light source itself can achieve increased emission efficiency. The laser exhibits high monochromaticity and thus enables color reproducibility to be enhanced. The laser eliminates the need for mercury, which is a hazardous substance.
As projectors that use a laser as a light source, small portable projectors with a brightness of about 10 lumens have been implemented. However, projectors with a brightness of about 10 lumens have difficulty allowing a further increase in the size of a projection screen while providing sufficient contrast when the surroundings of the projector are bright. Thus, for further increased projector brightness, development of a laser used as a light source and having an increased output level has been promoted.
On the other hand, when the laser is used as a light source, a laser safety standard needs to be satisfied which is specified in IEC (International Electrotechnical Commission) 60825-1, which is an international standard. IEC 60825-1 specifies an accessible emission limit (AEL) that is a laser safety standard for each of a plurality of laser classes 1, 2, 3R, and 3B. The accessible emission limit refers to the maximum accessible emission level of laser light that is permissible for each laser class according to the wavelength of the laser light and the emission duration of the laser light.
Thus, for projectors using a laser as a light source, the laser class also needs to be determined in accordance with the provision in IEC 60825-1. Furthermore, the maximum amount of luminous flux that can be projected by the projector can also be determined using the AEL value for each laser class. For example, the small portable projectors are roughly classified into a scan type that sequentially projects an image on a pixel-by-pixel basis and an LCOS type that projects all the pixels of the image when using an LCOS (Liquid-Crystal-On-Silicon) element, and it has been found that, for the same laser class 1 or 2, the LCOS projector has a larger maximum amount of projectable luminous flux than the scan type. This is because, for example, the LCOS type takes a longer image projection time than the scan type.
Hence, the LCOS projector can make the projection screen brighter than the scan projector. If a small LCOS projector is produced using a commercially available laser, this projector is roughly classified as laser class 1 or 2.
As described above, to improve brightness of the projector, development of a laser with an increased output level is being promoted. Even LCOS projectors are likely to be implemented which are classified in a laser class 3R or higher. In such as case, it is expected that the projector is designed so as to be classified as a predetermined laser class. Patent Literature 1 discloses a design technique for a projector that satisfies the safety standard for laser light sources. Moreover, the light source needs to be controlled such that the power of laser light emitted by the light source decreases to a value smaller than a rated value, so as to allow the laser class of the projector to be classified in a predetermined laser class.